The wavelength of light for lithography has been reduced into the deep ultraviolet (DUV) range to produce the feature size necessary for current and future electronics devices. The electronics industry is developing new resists that are tailored to the DUV range. One such resist class is chemically amplified resists.
The main components of chemically amplified resist formulations are a photoacid generator compound, a polymer resin and a solvent capable of dissolving the photoacid generator and the resin. For many positive chemically amplified resists, the polymer resin contains acid labile groups which make the polymer resin insoluble in an aqueous developer. Upon irradiation, the photoacid generator compound produces an acid which cleaves the acid labile groups resulting in a polymer resin that is aqueous soluble. Chemically amplified resists have generated a great deal of interest and there are numerous patents available discussing these compositions such as, for example, U.S. Pat. Nos. 5,069,997; 5,035,979; 5,670,299; 5,558,978; 5,468,589; and 5,389,494.
One group of polymers which can be used as resins in chemically amplified resists are acetal or ketal functionalized polymers. The alkali solubility of phenolic resins is greatly inhibited by converting the hydroxyl groups to acetal or ketal groups. Typically, acetal or ketal phenolic resins are produced by reacting a phenolic resin with a vinyl ether in the presence of an acid catalyst.
In addition, patents such as U.S. Pat. Nos. 5,670,259 to Imai et al. (Imai) and 5,714,559 to Schacht et al. (Schacht) disclose the use of cross-linked groups between two polymer chains. In Imai, the cross-linking groups are prepared by coating a substrate with (i) a polymer containing phenol or carboxyl groups and (ii) a compound with two to four vinyl ether groups. The substrate is then heated to form the cross-links between the polymer chains.
In Schacht, the acid labile cross-links are formed by reacting a polymer containing repeating units of hydroxystyrene and a vinylcyclohexanol in the presence of an acid catalyst to form acetal or ketal cross-links.
The acid-labile polymers are formulated with a photoacid generator compound to form a chemically amplified resist product. Upon irradiation, the generated acid cleaves the acid-labile protecting groups resulting in a photoresist which is soluble in an aqueous developer and thereby enhances the solubility of the polymer in the exposed areas without dissolving the unexposed areas.
The problem with the invention in Imai is that the amount of cross-linking is very sensitive to the baking conditions. If the baking conditions are not strictly controlled, the amount of cross-linking will be not be reproducible. This will change the dissolution and exposure characteristics of the resist resulting in a small lithographic process window.
In the Schacht patent, cross-linking between two phenol units of two different polymers was shown to be possible using poly(hydroxystyrene), monovinyl ether and an acid. However, the cross-linking between the two poly(hydroxystyrene) polymer chains is not an efficient and reproducible process in the presence of a monovinyl ether and an acid. Consistent cross-linking is necessary since important thermal and lithographic properties depend upon the reproducibility and extent of cross-linking.
Schacht also showed that cross-linking was formed between phenolic units and alcohol units, (such as cyclohexanol), in the presence of monovinyl ether and an acid. This mode of cross-linking requires a polymer of hydroxystyrene and hydroxycyclohexyl vinyl monomers. These type of copolymers are produced by partially hydrogenating the poly(hydroxystyrene) and such hydrogenation does not give reproducible concentrations of hydroxy-cyclohexyl moiety in the polymer. Therefore, the variability in the percentage of cyclohexyl moiety can dramatically influence the extent and reproducibility of cross-linking.
Another problem with acid labile, acetal-based resist is that the volatile by-products are formed after exposure to actinic radiation. These volatile by-products may coat the lens of the exposure tool resulting in reliability problems. In addition, volatile by-products cause the resist to shrink, which is not desirable for semiconductor manufacturing.
Therefore, it is an object of the present invention to provide a method of reproducibly preparing a partially cross-linked acid labile polymer suitable for use as a component in a chemically amplified photoresist. The photoresist will have high contrast, increased sensitivity and improved high temperature flow characteristics.
It is another object of this invention to provide a photoresist that has low volatility by-products when exposed to radiation, which will reduce the reliability problems of the exposure tool and results in less shrinkage of the resist.